1. Field of the Invention
The present invention relates to a nano semiconductor sheet, a method of manufacturing the nano semiconductor sheet, a thin film transistor (TFT), a flat panel display device, and methods of manufacturing a TFT and a flat panel display using the nano semiconductor sheet. More particularly, the present invention relates to a nano semiconductor sheet having nano particles arranged substantially in parallel, a method of manufacturing the nano semiconductor sheet, a TFT, a flat panel display device, and methods of manufacturing a TFT and flat panel display using the nano semiconductor sheet.
2. Description of the Related Technology
Thin film transistors (TFTs) are used for various electronic devices. Among other things, flat panel displays employ TFTs for various purposes. Examples of flat panel displays include liquid crystal display (LCD), organic light emitting display (OLED), and inorganic light emitting display. TFTs may serve as pixel TFTs, for example, switching devices or pixel driving devices. TFTs may also serve as circuit TFTs.
A TFT typically includes a semiconductor layer formed on a substrate. The semiconductor layer includes drain and source regions and a channel region disposed between the source and drain regions. The TFT also includes a gate insulation layer formed on the semiconductor layer, and a gate electrode formed on a portion of the gate insulation layer above the channel region.
An organic light emitting device typically includes a light emission layer. The emission layer is typically formed of an organic material and is interposed between an anode electrode and a cathode electrode. When positive and negative voltages are respectively applied to the electrodes, holes injected from the anode electrode move through a hole transport layer to the emission layer, and electrons injected from the cathode electrode move through an electron transport layer to the emission layer. The electrons and holes are recombined in the emission layer to generate excitons in their excited state. As the excitons return to a ground state from their excited state, fluorescent light is emitted from the emission layer. A full-color organic light emitting display includes pixels emitting three colors, such as red (R), green (G), blue (B), to realize a full color image.
An active matrix type organic light emitting display requires a high resolution panel. However, several problems may occur due to the use of the above-described TFTs when they include high performance polycrystalline silicon. In a conventional active matrix type flat panel display, for example, an active matrix type organic light emitting display, a circuit TFT and a pixel TFT (in particular, a driving TFT) may include the same type of polycrystalline silicon, and thus the TFTs may have the same current mobility. However, the circuit TFT requires switching characteristics whereas the driving TFT requires low current properties which are different from the switching characteristics. That is, when both a driving TFT and a circuit TFT in a high resolution display device include polycrystalline silicon films having a high current mobility, the circuit TFT can have a high switching characteristic. However, the current flowing to an electroluminescence (EL) device through the driving TFT increases, thus causing the luminescence to be too high and increasing a current density. These problems may reduce the lifetime of the EL device.
In certain display devices, a driving TFT and a circuit TFT in a display device may be formed using an amorphous silicon film having a low current mobility. In such cases, the driving TFT may be formed to decrease the amount of a current flowing through the driving TFT to EL device, whereas the circuit TFT may be formed to increase the amount of a current flowing through the circuit TFT.
In order to solve this problem, a method of adjusting the current flowing through the driving transistor has been proposed. For example, a method of increasing the resistance of channel regions by decreasing a width-to-length ratio (W/L) of the driving transistor has been attempted. In addition, a method of increasing the resistance of channel regions by forming low concentration doped regions in source/drain regions in the driving transistor has been proposed. However, when the channel length is increased to decrease the W/L ratio, a stripe pattern is formed in the channel region and the area of an aperture decreases when crystallization is performed using excimer laser annealing (ELA). When the width is decreased to decrease the W/L ratio, there is a limitation due to the design rule of a photolithographic process and the reliability of a transistor is degraded. The method of increasing the resistance of channel regions by forming low concentration doped regions requires an additional doping process.
Meanwhile, a flat panel display may be deformed to a predetermined degree by applying a tensile force to obtain a sufficient viewing angle. A flat panel display may also be used for portable products such as arm bands, wallets, or notebook computers. Thus, there is a need to provide flat panel displays with improved flexibility.
However, when polycrystalline silicon TFTs are formed using conventional methods, flat panel display may not be made flexible. To manufacture flexible products, most components including a substrate may be formed of flexible materials, such as acryl, polyimide, polycarbonate, polyester, mylar, or other plastic materials. However, these plastic materials are susceptible to heat.
Accordingly, to manufacture TFTs for flat panel displays for flexible products, a structure which can be manufactured at a temperature which plastic materials can endure and a method performed at the temperature are required.